Optical spectroscopy systems and wavelength division multiplexing (WDM) communication all rely on using spectrally dispersive element to direct optical signal corresponding to its wavelength. A most common practice is to spatially disperse signal using a grating, and then detected by an array of detectors. The use of the grating, however, requires system tradeoffs between sensitivity and resolution. This because the grating spectral resolution requires the use of a light blocking aperture that reduces the signal intensity. Moreover, a grating based spectrometer is large in size and not suitable for handheld instruments such as cell phones. Another approach widely used in telecommunication is the use of discrete thin film filters in combination with optical switches to achieve WDM channel selection. One drawback of this approach is the high cost that has prevented wide adoption at user terminals or in data centers with large amount of fiber connection nodes.
The incorporation of Fabry-Perot optical filters with microelectromechanical systems (MEMS) technology has enabled the realization of miniaturized optical systems for spectral filtering applications, including wavelength division multiplexing in fiber optical communications, hyperspectral imaging, and gas sensing spectroscopy. MEMS devices are compatible with semiconductor batch processes that precisely produce optical devices in large quantity at low cost. Fabry-Perot filters are composed of two parallel mirrors separated by an optical cavity. The light transmitted through such a filter is maximized at wavelengths of light that interfere constructively within the cavity between the mirrors. By altering the separation between the mirrors, the chosen order can be swept over a range of wavelengths, realizing a tunable optical filter. For spectroscopy applications relatively large apertures, a large tunable wavelength range and narrow line widths are required to directly place the filter in front of a detector. A number of MEMS-based Fabry-Perot filters have been fabricated for use in these applications, with most of these filters using electrostatic actuators to control the position of the movable mirror. To tune over a large wavelength range, the actuator must move the mirror over a relatively large distance. With electrostatic actuators, this is problematic. Electrostatic actuators are based on attraction between two oppositely charged surfaces that are separated by a tiny gap. To move a heavy load of an optical quality mirror of relatively large size, the actuator device must be operated under high electrical fields due to the weak force nature associated with electrostatics. Consequently, the system is nonlinear, with instability of the pull-in, thus intrinsically prone to failures including stick, wear, dielectric changing, and breakdowns. Moreover, the electrostatic device is also sensitive to moisture and requires expensive hermetic sealing. Additionally, the device has charge building-up induced long term drift problem. These deficiencies have prevented the wide use of MEMS based tunable optical filters.